This invention relates to a demodulator for demodulating a received signal into a demodulated signal by differential detection.
It is known in the art that a demodulator demodulates a received signal into a demodulated signal by differential detection. Such a demodulator may be used in a mobile communication system. The received signal is transmitted as a transmission signal from a transmitter. For example, the transmission signal is produced by differential phase shift keying in the transmitter. In this case, the transmission signal has phase information.
A conventional demodulator comprises a phase variation producing section and a demodulator section. The phase variation producing section is supplied with the received signal and produces a phase variation signal representative of a phase variation between an instantaneous phase based on the phase information and a phase offset determined by the demodulator. The phase variation signal is supplied to the demodulator section.
The demodulator section gives a predetermined delay time to the phase variation signal to produce a delayed phase variation signal. Furthermore, the demodulator section carries out differential detection in accordance with the phase variation signal and the delayed phase variation signal to produce the demodulated signal.
Inasmuch as differential detection is carried out in accordance with the phase variation signal and the delayed phase variation signal in the conventional demodulator, a symbol error rate increases in comparison with coherent detection. In order to improve the symbol error rate in the conventional demodulator, it is necessary to produce the transmission signal in using an error-correcting code.
However, transmission rate is decreased on using the error-correcting code as known in the art. Furthermore, it is necessary for the transmitter to have a coding circuit for carrying out error correction coding algorithm. In addition, it is necessary for the conventional demodulator to have a decoding circuit for carrying out decoding. Namely, not only the transmission rate decreases but also demodulator must have a complex structure on using the error-correcting code.